Add presubmit check to prevent package boundary violations.

tools-webrtc/testdata/multiple_errors_single_target/subpackage/BUILD.gn

Index: tools-webrtc/testdata/multiple_errors_single_target/subpackage/BUILD.gn
diff --git a/tools-webrtc/testdata/multiple_errors_single_target/subpackage/BUILD.gn b/tools-webrtc/testdata/multiple_errors_single_target/subpackage/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..700508155b62c1f98388fb46f0b73f36d4885667
--- /dev/null
+++ b/tools-webrtc/testdata/multiple_errors_single_target/subpackage/BUILD.gn
@@ -0,0 +1,14 @@
+# Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
+#
+# Use of this source code is governed by a BSD-style license
+# that can be found in the LICENSE file in the root of the source
+# tree. An additional intellectual property rights grant can be found
+# in the file PATENTS.  All contributing project authors may
+# be found in the AUTHORS file in the root of the source tree.
+
+group("dummy_supbackage") {
+  sources = [
+    "dummy_subpackage.cc",
+    "dummy_subpackage.h",
+  ]
+}